HLW canister array was designed by FLLSSM program, considering the thermal concentration. Rock properties were chosen as those of granite, the most possible bedrock for the repository in Korea. It was shown that repository area and excavation volumes can be determined by the pitch or distance between canisters. Pitch can be reduced to 0.6 m assuming the tolerance temperature as 200$^{circ}C$. Thermal concentration was reduced as storage time for cooling the canister passed. After 10 years of storage the thermal problems seemed to be negligible.